Excessive fluid can accumulate in patients suffering from end stage renal disease (ESRD) or congestive heart failure (CHF). The excess fluid first accumulates in the blood and expands the volume of blood leading to hypertension and places increased stress on the heart. This added stress often leads to heart failure and death. The fluid also can accumulate in the pleural cavities of the lungs leading to shortness of breath. Oxygen uptake in the lungs is reduced as air becomes displaced by water. Again, if this condition is not reversed, death can result.
According to the National Kidney Foundation, 20 million people have Chronic Kidney Disease (CKD) in the US, which is one in every nine Americans. The most severe stage of CKD, when kidneys cease to function, is End Stage Renal Disease (ESRD). According to the USRDS 2005 Annual Data Report, 452,957 people had ESRD in the US in 2003 and, of these, there were 324,826 prevalent dialysis patients. The mortality rate of ESRD patients who receive traditional hemodialysis therapy is 24% per year. The leading causes of death in patients with ESRD are cardiac related which accounts for 43% of all deaths in this population. In ESRD patients, fluid accumulates because their kidneys no longer can effectively remove the water and other fluids, which are consumed daily. The fluid accumulates first in the blood where the blood volume can expand by as much as 20%. The fluid then accumulates throughout the body ending up in the extremities such as the ankles, hands, and other tissues as edema (swelling). Volumes as large as 7-10 liters or about 15-20 pounds can commonly accumulate. This causes increased stress on the heart as evidenced by significant increases in blood pressure or hypertension and subsequent heart failure. About 60% of hemodialysis patients have chronic hypertension as defined by the World Health Organization (WHO) guidelines.
This fluid overload volume can only be removed from ESRD patients by direct ultrafiltration or by the ultrafiltration action of a dialysis procedure, generally carried out weekly in three 4 hour sessions. Removal of the large amounts of water in severe cases of fluid overload often causes fatigue and nausea and, in some cases, arrhythmias, “crashing,” and heart failure.
The fluid begins to re-accumulate again once the dialysis session is over. To minimize the fluid accumulation, severe fluid intake guidelines have been established for these patients. Frequently because of continual thirst, however, these fluid restrictions are not complied with because of the hardship they impose on the quality of life of these patients.
After the excess fluid has been removed and the proper blood volume has been obtained, blood pressure will drop and the cardiac stress will be reduced. However, repeated increases and decreases in blood volume may also eventually lead to damage to the heart and vascular system, thus further increasing the risk of cardiac disease. As re-accumulation of water occurs when the patient is not on the machine in a relatively short period of time, hypertension is nearly always present in hemodialysis patients to some degree. For those patients with residual kidney function, this chronic hypertension may cause rapid decay of this residual kidney function leading to the high mortality rates of the general ESRD population rather than the lower mortality rates of those ESRD patients with some residual kidney function.
The incidence of advanced CHF continues to grow and has become a disease of epidemic proportions throughout the world. According to the National Health and Nutrition Examination Surveys, an estimated 4.8 million Americans have CHF. In CHF patients, there is a progressive deterioration of the heart muscle that leads to an inability to pump enough blood to support the vital organs. As a result, fluid retention occurs because the blood perfusion pressure in the kidneys is reduced and the kidneys become inefficient in removing fluid.
While fluid overload in CHF patients can often be treated with numerous pharmacological agents, these drugs become gradually ineffective over time and may also cause undesirable effects such as kidney failure. There continues to be a growing body of literature that supports the concept of physically removing the fluid by blood ultrafiltration, which has been shown to improve patient outcomes and shorten hospital stays and intensive care unit utilization. In fact, fluid removal may be superior to the administration of very large doses of diuretic drugs.
There are several advantages to treating CHF fluid overload patients with ultrafiltration over diuretic drugs. Ultrafiltration offers an efficient fluid removal without those side effects seen with drugs such as kidney failure and blood pressure drops. Furthermore, ultrafiltration quickly relieves the symptoms of shortness of breath and joint swelling.
Ultrafiltration is a process by which blood is exposed (under pressure) to a semi-permeable membrane. The membrane properties dictate that water, salts, and other small molecular weight molecules pass through the membrane, but blood cells, proteins, and other large molecular weight molecules are not separated. The ultrafiltration cartridge is generally made up of a very large number of small diameter hollow fiber membranes. Typically, blood is removed from the patient via a catheter placed in an artery or large vein and is pumped into the ultrafiltration cartridge to generate the pressure necessary to carry out the ultrafiltration process. The hollow fibers are arranged so that the blood is perfused through these hollow fiber membranes and the filtered fluid is then removed and discarded, while the treated blood is then returned via another catheter back to the patient.
Conventional ultrafiltration devices have several drawbacks. The procedures are carried out on machines that must be plugged into an electrical circuit and therefore the patients have limited mobility during the typically thrice weekly, 4-hour procedures. Because ultrafiltration is generally carried out during a standard dialysis session, the excessive water volume must be removed in this 4-hour period, which places additional physiological burdens on the patients.
Because of the close relationship between blood volume and blood pressure, there is an additional complication using conventional ultrafiltration procedures related to total amount of fluid removed during a typical session. The fluid amount to be removed is generally determined by the amount of weight the dialysis patient has gained since the last dialysis and/or ultrafiltration session. Excessive fluid removal often leads to a significant drop in the patient's blood pressure (hypotension), which can lead to hemodynamic instability and fainting, cardiac arrest, or death.
There is an increasing body of evidence that continuous removal of accumulated water through daily home dialysis or continuous ambulatory peritoneal dialysis (CAPD) results in significantly improved patient outcomes and far fewer physiological burdens being placed on the patients. However, the complexity and immobility of home dialysis procedures as well as the medical complications, such as infection and scarring, associated with long-term peritoneal dialysis, severely restricts the use of these ultrafiltration procedures to effectively treat hypervolemia.
Another drawback of conventional ultrafiltration is the need to use anticoagulants, such as heparin or citrate, to prevent the blood from clotting in conventional ultrafiltration devices. In order to adapt conventional ultrafiltration devices for continuous use, continuous anticoagulation must be utilized at anticoagulant levels sufficient to prevent clots from forming in the device. Prolonged use of anticoagulants presents a significant risk to patients in general because of the possibility of uncontrolled bleeding occurring and particularly to the majority of ESRD patients who are undergoing thrice weekly hemodialysis procedures during which they also receive anticoagulation.
An additional drawback of the adaptation of conventional ultrafiltration to the continuous treatment of hypervolemia resides in the complications of blood access and the use of pumps. Most blood access for conventional ultrafiltration devices is carried out via indwelling venous catheters or arterio-venous fistulas in the case of certain ESRD patients. Notwithstanding the complications associated with the long term use of these blood access devices, they require the use of special blood pumps in the extracorporeal circuit in order to generate the flow rates and perfusion pressures required to achieve fluid removal in the ultrafiltration device. Blood access catheters that are placed in high pressure arteries have been utilized to obviate the need for additional pumping mechanisms to achieve the blood flow rates and pressures required, but safety concerns for their use outside an intensive care environment render them impracticable.
The use of membrane-based ultrafiltration systems for the treatment of blood has been well documented in extracorporeal systems for over 30 years. However, the use of these systems for continuous applications has been hampered by a number of technical hurdles relating primarily to blood clotting and biocompatibility. Firstly, the cartridges contain a large number of small diameter hollow fiber membranes, which presents a large contact surface for filtration and toxin clearance. While this large surface area, approximately 1-2 m2 (10,000-20,000 cm2) is required to achieve the performance characteristics required for a short term (2-6 hr) extracorporeal ultrafiltration session, it exposes the blood to an equally large surface area of foreign material. The small diameter membranes are used to minimize the extracorporeal volume of blood that is required to be used during typical hemodialysis or ultrafiltration. This combination of large numbers of fibers coupled with their small diameters results in an overwhelming surface-to-volume ratio with which the natural coagulation system of the patient must deal. As a result, a high level of anticoagulation is required to prevent the blood from clotting in the cartridge. While this anticoagulation is medically acceptable over the relatively short period of the hemodialysis or hemofiltration sessions, long-term chronic use of high doses of anticoagulants is medically unacceptable. Even with the use of anticoagulation, continuous use in an extracorporeal circuit of existing dialyzers is generally not possible for more than approximately 48-72 hours.
This inherent thrombogenicity of the existing hollow fiber ultrafiltration devices is further complicated by the design of the inlet and outlet elements of the cartridges which are used in existing devices to (i) distribute blood from a single inlet conduit to the large number of hollow fiber membranes and (ii) to collect the blood from the large number of hollow fiber membranes and channel the blood to a single outlet conduit. These designs allow for a number of stagnation points within these elements of the cartridge increasing the thrombogenicity of existing devices. Furthermore, these elements do not distribute the blood uniformly to all the hollow fiber membranes resulting in significant differences in blood velocity and performance within different areas of hollow fiber membranes.
Secondly, long-term blood access continues to be problematic. Percutaneous catheter use in hemodialysis patients is plagued with issues related to bleeding, infection, and clotting that require a high level of attention to maintain these blood conduits patent for use. There have been some recent developments in catheter design that may improve these catheters, but currently they are unsatisfactory for long-term use due to the persistence of the previously mentioned blood access problems.
The use of large bore, approximately 6 mm diameter, vascular grafts have been largely successful as a long-term blood access conduit in vascular reconstruction surgery. Graft survivals of over 5 years continuous use have been shown with the use of low or no anticoagulants. In a recent study of an implantable membrane device to be used as an artificial pancreas, a 60 cm long coil of 6 mm inner diameter hollow fiber ultrafiltration membrane was implanted into large animals by attaching the device directly to the circulatory system via 6 mm polytetrafluoroethylene (PTFE) vascular grafts as an arterio-venous shunt using the iliac artery and vein. These devices were found to remain patent for periods of up to 4 years without the need for systemic anticoagulation and the patency rate was similar to that found with the 6 mm PTFE graft alone.
The devices and techniques disclosed herein are designed to address these and other deficiencies of prior art devices and techniques for addressing hypervolemia in ESRD and CHF patients through continuous ambulatory volume control and addressing blood toxicity in renal failure patients through hemodialysis.